Field of the Invention
The present invention concerns a method to generate a tomosynthetic x-ray image of a breast of a patient, as well as a mammography apparatus operated with such a method.
Description of the Prior Art
Tomosynthesis is an imaging modality in which projection images (projection data sets) of an examination subject are acquired with a digital x-ray detector at a number of different projection directions. By known image reconstruction methods, a three-dimensional image data set can be generated from these digital projection images acquired from different projection directions or projection angles in a limited angle range (for example between −25° and +25° relative of the acquisition area of the x-ray detector), i.e. from the image data representing these projection images, this three-dimensional image data set is compose of a number of slice images that each depict a subject slice oriented parallel to the acquisition area of the x-ray detector. Tomosynthesis is in particularly useful in mammography to generate three-dimensional x-ray images of the breast.
From DE 10 2005 022 543 A1 and from M. D. Hoernig et al., “Design of a contrast-enhanced dual-energy tomosynthesis system for breast cancer imaging”, Medical Imaging 2012, Proceedings of the SPIE, Vol. 8313, article id. 831340, a contrast-enhanced digital tomosynthesis method is known in which a contrast agent is intravenously injected into the patient before the data acquisition, which contrast agent propagates in the blood vessels of the breast. A first tomosynthetic scan with a low-energy x-ray beam is subsequently implemented, followed by a second tomosynthetic scan with a high-energy x-ray beam. The energy spectrum of the low-energy x-ray beam is thereby selected so that the contrast agent is practically invisible in the respective projection images obtained in that low-energy scan, while the high-energy x-ray beam is strongly absorbed by the contrast agent (for example iodine). From the tomosynthetic 3D x-ray images or volume data sets that are generated in this way, and by a weighted subtraction, a tomosynthetic difference image is generated from which healthy tissue is largely eliminated, which would impair the ability to detect a tumor or a microcalcification. However, in the known procedure it is disadvantageous that slice images that are already plagued with a loss of information (due to the calculation steps taken within the scope of the tomosynthetic reconstruction, for example the use of spectral filters and the dynamic compression given filtered back-projection) are used for the weighted subtraction.
In order to avoid these disadvantages, it is possible before the tomosynthetic reconstruction, to generate a composite projection image from the projection images acquired at a respective angle position, but acquired at different x-ray energies by a weighted subtraction, and then to reconstruct the tomosynthetic 3D x-ray image from such composite images. However, a problem with such a procedure is that the tomosynthesis scan is implemented during a continuous pivot motion of the x-ray tube, such that the projection angles of the first tomosynthesis scan deviate slightly in practice from the projection angles of the second tomosynthesis scan, due to unavoidable tolerances (mechanism, pulse time, timing of the x-ray detector). This angle deviation is typically approximately 0.2 to 0.6°. Thus, in practice, it is not possible to reliably produce the aforementioned composite image by a subtraction of two images respectively acquired at precisely the same projection angle.